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New solid-construction sandwich wall element

Composite thermal insulation systems in the solid-construction area are possible as sandwich wall elements consisting of a load-bearing supporting shell of reinforced concrete, a thermal insulation layer of rigid foam and a facing shell. Connection between the individual layers is usually by means of mechanical fasteners. The elimination of these fasteners results in economic advantages in terms of material savings and manufacturing complexity as well as improved thermal resistance with a lower component thickness.

For many years, sandwich elements made of sheet steel with a core of rigid foam have been recognised structures in building facades. In this research project, the direct application of the thermal insulation layer onto the two steel sheets without mechanical fasteners during the manufacturing process is transferred to concrete and glass cover layers. The concrete as the supporting shell takes care of vertical and horizontal load transfer and the rigid foam takes care of thermal insulation and load transfer from the glass facing shell used for weather protection and facade design. The materials used are normally-cooled float glass made of soda lime silicate glass, rigid polyurethane foam and ultra-high-strength concrete.

The elimination of mechanical fasteners requires reliable knowledge of the bonding behaviour of the intermediate layer of rigid foam and the concrete as well as the glass. The transfer of tensile and shear forces resulting from the dead load of the glass sheets and the insulation layer, from the wind load or from forced loading, takes place in the boundary layers between the three materials exclusively via the adhesive bond. The initial aim of the research project was therefore to demonstrate the technical feasibility of the glass rigid-foam concrete sandwich elements and to investigate the transmission of adhesive tensile and shear forces in the composite joint. With regard to the manufacturing process, the mechanically prefabricated semi-finished products made of glass and insulating material are then in a further step concreted horizontally. Adhesive tensile and shear tests with different surfaces in the composite joint between the glass and insulating material as well as between the concrete and insulating material were carried out on small test specimens produced using this procedure. The electron microscopic analysis of the fracture surfaces shows that the adhesion promoter positively influences the formation of pores immediately after application of the liquid polyurethane mass to the glass surface. The rigid foam processed with an adhesion promoter has significantly smaller pores in higher numbers at the interface to the glass. Compared to rigid foam without adhesion promoter, this results in a larger effective area for the transfer of adhesive tensile and shear forces, which leads to higher strengths between the glass surface and the rigid foam.

The research of the University of Siegen shows the technical feasibility of the glass rigid-foam concrete sandwich elements as well as the transmission of adhesive tensile and shear forces in the composite joint. However, further tests on the load-bearing behaviour are required that take into account the larger dimensions of the test specimens as well as additional mechanical and, in particular, climatic stresses. The aim of the subsequent investigations will be to develop and realise a prototype as a basis for industrial production.

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